To distribute supply air throughout a building, fabric air ducts are often preferred over sheet metal ones for various reasons. Fabric ducts can evenly distribute the air, can be less prone to surface condensation, have a nice appearance, are lightweight, and are usually removable for periodic cleaning. Fabric ducts, nonetheless, do have some drawbacks.
Fabric ducts, for instance, can sometimes be difficult to remove and attach to a main supply air duct. In some cases, the point of attachment is at a location with limited accessibility, particularly if the main supply air duct is directly above the fabric duct.
Some fabric ducts have rigid pieces that are unsuitable for cleaning by conventional fabric washing machines (e.g., item 60 of U.S. Pat. No. 6,280,320 and item 24 of published U.S. Patent Application No. 2002/0155805).
In some applications, it is desirable to have an air duct made of a fabric that is more air permeable in some areas than others. For example, there might be a need for more airflow below the duct than above it. This could be accomplished by joining fabrics of different permeability; however, sewing together different materials does not always work well.